.\" $OpenBSD: PKCS7_sign.3,v 1.13 2020/06/10 11:43:08 schwarze Exp $
.\" full merge up to: OpenSSL df75c2bf Dec 9 01:02:36 2018 +0100
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.Dd $Mdocdate: June 10 2020 $
.Dt PKCS7_SIGN 3
.Os
.Sh NAME
.Nm PKCS7_sign
.Nd create a PKCS#7 signedData structure
.Sh SYNOPSIS
.In openssl/pkcs7.h
.Ft PKCS7 *
.Fo PKCS7_sign
.Fa "X509 *signcert"
.Fa "EVP_PKEY *pkey"
.Fa "STACK_OF(X509) *certs"
.Fa "BIO *data"
.Fa "int flags"
.Fc
.Sh DESCRIPTION
.Fn PKCS7_sign
creates and returns a PKCS#7 signedData structure.
.Fa signcert
is the certificate to sign with,
.Fa pkey
is the corresponding private key.
.Fa certs
is an optional additional set of certificates to include in the PKCS#7
structure (for example any intermediate CAs in the chain).
.Pp
The data to be signed is read from
.Vt BIO
.Fa data .
.Pp
.Fa flags
is an optional set of flags.
.Pp
Any of the following flags (OR'ed together) can be passed in the
.Fa flags
parameter.
.Pp
Many S/MIME clients expect the signed content to include valid MIME
headers.
If the
.Dv PKCS7_TEXT
flag is set, MIME headers for type
.Sy text/plain
are prepended to the data.
.Pp
If
.Dv PKCS7_NOCERTS
is set, the signer's certificate will not be included in the PKCS7
structure, though the signer's certificate must still be supplied in the
.Fa signcert
parameter.
This can reduce the size of the signature if the signer's certificate can
be obtained by other means: for example a previously signed message.
.Pp
The data being signed is included in the
.Vt PKCS7
structure, unless
.Dv PKCS7_DETACHED
is set, in which case it is omitted.
This is used for PKCS7 detached signatures which are used in S/MIME
plaintext signed messages for example.
.Pp
Normally the supplied content is translated into MIME canonical format
(as required by the S/MIME specifications).
If
.Dv PKCS7_BINARY
is set, no translation occurs.
This option should be used if the supplied data is in binary format;
otherwise, the translation will corrupt it.
.Pp
The signedData structure includes several PKCS#7 authenticatedAttributes
including the signing time, the PKCS#7 content type and the supported
list of ciphers in an SMIMECapabilities attribute.
If
.Dv PKCS7_NOATTR
is set, then no authenticatedAttributes will be used.
If
.Dv PKCS7_NOSMIMECAP
is set, then just the SMIMECapabilities are omitted.
.Pp
If present, the SMIMECapabilities attribute indicates support for the
following algorithms: triple DES, 128-bit RC2, 64-bit RC2, DES
and 40-bit RC2.
If any of these algorithms is disabled then it will not be included.
.Pp
If the flags
.Dv PKCS7_STREAM
is set, then the returned
.Vt PKCS7
structure is just initialized ready to perform the signing operation.
The signing is however
.Sy not
performed and the data to be signed is not read from the
.Fa data
parameter.
Signing is deferred until after the data has been written.
In this way data can be signed in a single pass.
.Pp
If the
.Dv PKCS7_PARTIAL
flag is set, a partial
.Vt PKCS7
structure is output to which additional signers and capabilities can be
added before finalization.
.Pp
If the flag
.Dv PKCS7_STREAM
is set, the returned
.Vt PKCS7
structure is
.Sy not
complete and outputting its contents via a function that does not
properly finalize the
.Vt PKCS7
structure will give unpredictable results.
.Pp
Several functions including
.Xr PKCS7_final 3 ,
.Xr SMIME_write_PKCS7 3 ,
.Xr PEM_write_bio_PKCS7_stream 3 ,
and
.Xr i2d_PKCS7_bio_stream 3
finalize the structure.
Alternatively finalization can be performed by obtaining the streaming
ASN.1
.Vt BIO
directly using
.Fn BIO_new_PKCS7 .
.Pp
If a signer is specified, it will use the default digest for the
signing algorithm.
This is
.Sy SHA1
for both RSA and DSA keys.
.Pp
In OpenSSL 1.0.0, the
.Fa certs ,
.Fa signcert ,
and
.Fa pkey
parameters can all be
.Dv NULL
if the
.Dv PKCS7_PARTIAL
flag is set.
One or more signers can be added using the function
.Xr PKCS7_sign_add_signer 3
and attributes can be added using the functions described in
.Xr PKCS7_add_attribute 3 .
.Xr PKCS7_final 3
must also be called to finalize the structure if streaming is not
enabled.
Alternative signing digests can also be specified using this method.
.Pp
In OpenSSL 1.0.0, if
.Fa signcert
and
.Fa pkey
are
.Dv NULL ,
then a certificate-only PKCS#7 structure is output.
.Pp
In versions of OpenSSL before 1.0.0 the
.Fa signcert
and
.Fa pkey
parameters must
.Sy NOT
be
.Dv NULL .
.Sh RETURN VALUES
.Fn PKCS7_sign
returns either a valid
.Vt PKCS7
structure or
.Dv NULL
if an error occurred.
The error can be obtained from
.Xr ERR_get_error 3 .
.Sh SEE ALSO
.Xr PKCS7_add_attribute 3 ,
.Xr PKCS7_encrypt 3 ,
.Xr PKCS7_final 3 ,
.Xr PKCS7_get_signer_info 3 ,
.Xr PKCS7_new 3 ,
.Xr PKCS7_sign_add_signer 3 ,
.Xr PKCS7_verify 3
.Sh HISTORY
.Fn PKCS7_sign
first appeared in OpenSSL 0.9.5 and have been available since
.Ox 2.7 .
.Pp
The
.Dv PKCS7_PARTIAL
and
.Dv PKCS7_STREAM
flags were added in OpenSSL 1.0.0.
.Sh BUGS
Some advanced attributes such as counter signatures are not supported.
